There are various types of compound semiconductor devices. Among them, a high electron mobility transistor (HEMT) has advantages of low noise and high speed operation, and is used in an application such as a base station for mobile telephones by exploiting these advantages.
In the HEMT, a two-dimensional electron gas induced in an electron transit layer serves as a carrier. While there are various materials for the material of the electron transit layer, the use of a nitride semiconductor material such as GaN as a material for the electron transit layer can enhance a withstand voltage of the HEMT by a large bandgap inherent to the nitride semiconductor.
In order to induce the two-dimensional electron gas in a GaN layer of the electron transit layer, an AlGaN layer which has different lattice constant and different spontaneous polarization than those of the GaN layer may be formed on the GaN layer. In this case, distortion occurs in AlGaN layer due to the lattice constant difference between the layers. Due to this distortion, piezoelectric polarization and spontaneous polarization occurs in AlGaN layer, which in turn induces the two-dimensional electron gas in the GaN layer of the electron transit layer.
However, under some circumstances, a current collapse phenomenon occurs in the HEMT that uses nitride semiconductor.
The current collapse phenomenon is a phenomenon in which on-resistance is raised along with an increase in source-drain voltage, and a drain current is little increased as a consequence. The current collapse phenomenon is one of factors that inhibit the higher output of the HEMT.